What other colors are in the room? Natural light? How much floor is there? What is there now (color and type)?

Classic medium tan like the lighter tans in the big tiles on the wall. Match the new floor tile grout to the wall tiles' grout color. Look at Daltile for a wide range of colors. https://www.daltile.com/how-to/how-to-choose-the-right-tile/floor-tile

My second choice would be a dark chocolate brown but it will show dust and hair. If there's a lightly mottled dark chocolate brown, it won't show dust or hair nearly as fast as plain brown.

I’d use beige to make the bathroom feel lighter and larger. Can’t recommend size or content, as it depends on the size of the bathroom and your budget. Be sure to buy a box (that can be returned) and lay out a few tiles to get a sense of its outcome. Add light towels/shower curtain as well.

Either of these terrazzo options from Zia Tile. 12 x 12 for a lighter lift to the room.

https://www.ziatile.com/products/apollo-12x12?nbt=nb%3Aadwords%3Ax%3A18064826366%3A%3A&nb_adtype=pla&nb_kwd=&nb_ti=&nb_mi=126347071&nb_pc=online&nb_pi=shopify_US_10517409759425_48530748276929&nb_ppi=&nb_placement=&nb_li_ms=&nb_lp_ms=&nb_fii=&nb_ap=&nb_mt=&gad_source=1&gad_campaignid=17336361303&gbraid=0AAAAADHhcIlL6tEjyWVrjaDMfJW8rfsoE&gclid=EAIaIQobChMIpY_cm9mwjwMVSOTCBB0ljS31EAQYBCABEgJL-fD_BwE

Can we see the bathroom?